Introduction sentence startersWatch
'The Conservative party were in government from 1951-64, winning 3 elections in this period. They had four different party leaders in this time - Churchill, Eden, MacMillan and Douglas-Home - who each served as Prime Minister.'
Then I'd introduce the possible different explanations for their dominance, so...
'Historians have offered many different explanations for the Conservatives' dominance of British politics in this period, including Factor X, Factor Y and Factor Z.'
Then, depending on whether your teacher/school like you to have a firm argument throughout or whether they only want you to choose the most important factor in the conclusion, you could say...
'In this essay, I will argue that Factor X was the primary reason, for reasons A and B.' (briefly outline your reasoning)
'In this essay, I will argue that Factors X, Y and Z all played a significant role, and their accumulative effect greatly increased the Conservatives' dominance in the years 1951-64.'
Just follow a formula like this each time, and they'll take you no time at all.